This invention relates generally to dispersion management in fiber optic systems and, more particularly, to the use of microstructure optical fibers (MOFs) to achieve dispersion compensation.
High bit rate optical transmission systems require periodic dispersion compensation to correct for the pulse broadening and distortion that occurs due to the dispersion of the transmission fiber. The accumulated dispersion over multiple amplifier spans can often limit the maximum data transmission rate. See, Agrawal, Fiber Optic Communication Systems, Ch. 9, John Wiley and Sons (1997), which is incorporated herein by reference. To minimize the limitation of system performance due to dispersion, a single wavelength system can be made to operate near the zero-dispersion wavelength of the transmission fiber span. For state-of-the-art multiple wavelength systems, this type of operation is not possible due to the broadband spectral transmission and the detrimental nonlinear effects that occur between multiple wavelengths when operating near the zero-dispersion wavelength of a long fiber span. In order to avoid serious nonlinear impairments, fiber designed for dense wavelength division multiplexed (DWDM) systems in the 1550 nm range typically have dispersion values D between +2 and +6 ps/nm-km for terrestrial systems and between xe2x88x922 to xe2x88x9210 ps/nm-nkm for undersea systems. In a terrestrial system the accumulated dispersion over each transmission span is typically compensated through the use of a section of dispersion compensating fiber that provides net dispersion of equal magnitude and of opposite sign to the dispersion of the span. Dispersion compensating fiber (DCF) with an anomalous dispersion value D xcx9cxe2x80x9480 ps/nm-km at 1550 nm is easily achieved requiring only about 10 km of DFC compensate for an 80 km transmission span. See, Vengsarkar et al., Optics Lett., Vol. 18, No. 11, pp. 924-926 (Jun. 1993), which is also incorporated herein by reference.
On the other hand, the maximum positive dispersion value that can be achieved in standard silica optical fibers is limited to the value of silica material dispersion, which ranges from 0 ps/nm-km at xcx9c1290 nm to 20 ps/nm-km at 1550 nm. Conventional optical fibers can be designed such that the net dispersion can be significantly lower than this value but not higher. This limitation is inherent in standard fibers used to compensate for fiber spans where the accumulated dispersion D is negative. Silica fiber can compensate only for wavelengths above xcx9c1290 nm, and long lengths of fiber would be necessary due to the relatively small maximum value of positive dispersion that can be achieved. Fiber that is presently being installed for use in DWDM systems at 1550 nm has normal (negative) D below xcx9c1450 nm, and there are not practical broadband fiber solutions for dispersion compensation at shorter wavelengths. For example, a system operating at 1350 nm over an 80 km span of TrueWave(copyright) (a trademark of Lucent Technologies Inc.) fiber with D=xe2x88x9210.5 ps/nm-km would require over 40 km of standard fiber to compensate for the accumulated dispersion, and it can not compensate for dispersion slope.
Microstructure optical fibers (MOFs) have recently been shown to exhibit large values of anomalous dispersion (positive D) for wavelengths above xcx9c700 nm with peak values greater than 100 ps/nm-km for simple structures. See, U.S. Pat. No. 6,097,870 filed on May 17, 1999 and issued on Aug. 1, 2000 to J. K. Ranka and R. S. Windeler (hereinafter the Ranka-Windeler patent) and J. K. Ranka et al., Optics Lett., Vol. 25, No. 1, pp. 25-27 (Jan. 2000), both of which are incorporated herein by reference. However, the dispersion of this type of MOF design at wavelengths above 1300 nm has not been discussed in the literature.
In accordance with one aspect of our invention, a fiber optic system comprises an optical transmitter, an optical receiver, and an optical fiber transmission path that optically couples the transmitter and the receiver to one another. The transmission path includes a first section that has negative dispersion at an operating wavelength xcex0 greater than about 1300 mn and a second section that includes a MOF. The MOF has relatively large anomalous dispersion at xcex0 and is sufficiently long to compensate the accumulated negative dispersion in the first section. In one embodiment the MOF comprises a core, a lower index cladding that includes one or more layers of air holes surrounding the core, characterized in that the diameter of the core is less than about 8 xcexcm and the difference in effective refractive index between the core and cladding is greater than about 0.1 (10%). Preferably, the cladding contains no more than 2 layers of air holes and the distance between the nearest edges of adjacent air holes is less than about 1 xcexcm. Although such a fiber would be multimode, light (i.e., optical radiation) entering the fiber would be launched into the fundamental mode and would remain guided in that mode.